The present invention relates generally to the field of watermarking. In particular, the present invention relates to methods, apparatus, and systems for signal continuity assessment using embedded watermarks.
Digital Watermarking systems are used in a variety of applications, including copy management, broadcast verification, integrity verification, and tamper detection. In certain applications, it may be desired to determine if a multimedia host signal, comprising audio, video, still images, text or other types of information, has been received in its entirely, in a desired sequence, without additional signal insertions or deletions. In addition, it may be desired to measure the extent of such reordering, insertions, or deletions in a version of the multimedia content, and to determine whether any such modifications were results of intentional signal tempering or were due to expected signal impairments that may occur during the normal course of signal processing and distribution through various communication channels. The measure of insertions, deletions and reordering can be used to assist in discriminating plagiarism or piracy attempts from fair content use, such as content sampling or spurious capture.
The use of watermarks for tamper detection is well documented in the prior art. A typical implementation involves the insertion of ‘fragile’ watermarks into the host signal. Any subsequent alterations of the host signal would either destroy, degrade or modify the embedded watermarks in a measurable way. Thus the integrity of a received host signal may be verified by detecting the presence and/or quality of the extracted watermarks. In some prior art publications, the embedded watermarks are designed in a way to enable the recognition of the type and amount of processing, or tampering, that has taken place. These fragile watermarks, however, may not be able to withstand significant amounts of host signal alterations and are inevitably destroyed by large signal distortions. In addition, they are not capable of entirely detecting modifications of signal continuity that is one of the objectives of the present invention. For example, an audio signal, containing embedded fragile watermarks, may be cut into several segments and transmitted in an out-of-order sequence with no other modifications. If these cuts are made at proper locations (e.g., along audio signal portions not containing watermarks such as silent intervals), the re-arranged fragile watermarks could remain intact and the tempering may remain undetected.
Another approach is to search for the continuous presence of embedded watermarks within a received host signal. However, simple continuity search may not be very effective since (a) the host content may not be able to accommodate continuous embedding of watermarks (e.g., due to perceptibility considerations), and (b) simple continuity check would not distinguish legitimate versus unauthorized signal alterations that result in host signal discontinuity. In general, signal continuity alterations, such as segment reordering, segment insertions or deletions, may be the result of intentional tempering, may be due to losses incurred in the transmission or storage of the host signal, or may be the result of inadvertent, but legitimate, acts of an authorized party. While, in all three cases, the altered multimedia signal generally contains the same type of impairments, different system reactions may be desired based on the source of such alterations. For example, in a Digital Rights Management (DRM) system that uses embedded watermarks to effect copy protection, an attacker may attempt to interfere with the detection of watermarks by reordering, cutting out or adding segments in the content. In this case, the desired system reaction may be to stop the playback, recording or transfer of the effected multimedia content in order to prevent the circumvention attempt. In another example, a copy protected movie, with a watermarked audio track, may be playing in the background of a birthday party while one of the participants makes a home video using a camcorder. The recorded home video may contain portions of the copy protected soundtrack, albeit in a fragmented format, with various deletions, additions or out-of-order sequences. In this scenario, a playback or recording device, which is equipped with a DRM compliant watermark detector, may be required not to interfere with the playback or recording of the home video. In yet another example involving a broadcast monitoring system, an embedded multimedia content may be transmitted through a noisy terrestrial broadcast channel and received at a monitoring station. In this case, some embedded watermarks may be lost due to inherent distortions of the transmission channel, resulting in a detected watermark sequence that resembles cuts, additions or out-of-order sequencing of the host content. The proper system reaction in this case may involve a best-estimate reconstruction of the detected watermark sequence in order to verify the start time, duration, and other pertinent information regarding the broadcast of a particular program. Furthermore, it may be desired to identify truncations, edits, or repeats of broadcast programming that may have taken place prior to the broadcast (but after the watermark embedding) of the host content. Therefore, it is not only necessary to detect discontinuities in a host signal but it is also important to identify candidate causes of such discontinuities in order to initiate an appropriate system response.
Differing system reactions to the detection of a discontinuous host signal could also create security loopholes since an attacker may alter the host content to mimic legitimate modifications. It is therefore important to provide the capability for identifying legitimate versus unauthorized alterations, or alternatively, to set limitations on the extent of allowable authorized modifications to a content. The methods, apparatus, and systems of the present invention provide the foregoing and other advantages.